Dual signal frequency transmission network



April 20, 1943. G. MOUNTJOY DUAL SIGNAL FREQUENCY TFEANSMISSION NETWORKFiled Jan. 7, 1941 INVENTOR r AHT'ORNEY W m h Patented Apr. 20,1943

UNITED STATES PATENT OFFlCE, nUaLsrGNsL ri t rzniairrnmsmssrorr I v vGarrard Mounuoy. Manhasset, N. Y, to Radio Corporation of America, acorporation of Delaware Application January 7, 1941, Serial no. erases(Ci. it's-44) y 6 Claims. My present invention relates to signaltransmission networks of the type adapted selectively to transmitbetween electron discharge transmission tubes at least two modulatedcarriers of different frequency values, and more particularly to anintermediate frequency carrier transmission standard broadcast waveswhich are of the ampli-,'

tude modulated carrier type. Such reception is in the- 550 to 1700kilocycle band, and each signal channel has a width of 10 kilocycles. Asis well known, each carrier frequency is of constant value, but ismodulated in amplitude by the modulating signals. The presently assignedfrequency modulation reception band, on the other a hand, comprisesthe-43 to 50 megacycl'e range,-

and utilizes a channel widthof 200 kilocycies. In this type of receptionthe carrien frequency is of constant amplitude, but the carrier variesin frequency in dependence upon the amplitude of the modulating signals.In the latter case the rate of carrier frequency deviation'depends uponthe frequencies of the modulation signals themselves.

Present broadcast receivers adapted to. receive both amplitude andfrequency modulated carrier waves are of the superheterodyne type. It isdesirable in such receivers to utilize the intermediate frequencyamplifier tubes located between the converter and second detector stagesfor both types of reception. There then arises the problem of providingnetworks, between the cascaded tubes following the converter, which arecapable of transmitting with efficiency the frequency modulatedcarrier-waves whose center frequency has a value of say 4.3 megacycles.In amplitude modulated carrier reception the intermediate frequencyvalue is usually 455 kilocycles. 1

Accordingly, it may be stated that it is one of the main objects of mypresent invention to provide between the source of amplitude orfrequency modulated carrier waves and an amplifier tube a'network whichcomprises a resonant circuit tuned to the center frequency of theamplitude modulated waves, and such resonant circuit acting as thecoupling'means between the source and amplifier; whereas for frequencymodulated carrier wave reception the resonantcir'. cult functions toprovide the reactive coupling between a pair of resonant circuitseachtuned to the center frequency of the frequency-modw lated waves.

Another important object of the invention is to provide between theconverter stage of a superheterodyne receiver, arranged for amplitudeand frequency modulation reception, and at least one stage ofintermediate frequency amplification, a

coupling network which comprises for amplitude reception a resonantcircuit tuned to a low intermediate frequency, whereas for frequencymodulation reception the network comprises a pair of resonant circuitstuned to a high inter'-' mediate frequency value, and the singleresonant circuit then functioning to provide the'sole capacitativecoupling between the pair of higher intermediate frequency resonantcircuits.

Still other objects of the invention are to improvegenerally thesimplicity and efficiency of coupling networks in the intermediatefrequency stages of a superheterodyne receiver adapted forselectivelyreceiving amplitude and frequency modulated carrier waves, and moreespecially to provide such coupling networks in an economical manner.

The novel features which I believe to be 'characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawingin which I have indicated diagrammaticallya circuit organization whereby my invention may be carried into effect.

In the drawing:

Fig. 1 illustrates an embodiment of the invention arranged between theconverter of a superheterodyne receiver and the first intermediatefrequency amplifier tube,

" Fig. 2 shows. the resonance curve of thenetwork Is-Giwhen utilized asthe sole coupling network, n Fig.3 shows the resonance curve of theentire network, shown in Fig. 1, when receiving frequency modulatedcarrier waves. I

Referring, now, to Fig. 1 there is only shown my present inventionapplied between thetconverter stage and the first intermediate amplifiertube. It is not believed necessary to show any circuit details of theconverter 'networlnnor of the stages preceding the latter, since thoseskilled in the art are fully acquainted with the manner of constructingthese circuits, and, further, because the present invention is confinedto the coupling network itself. Furthermore, it is not believednecessary to show the circuit details of any of the stages following thecoupling net work for the same reason. In general, it will. be

understood that the converter stage may, for

example, be of the combined local oscillator-first detector type. Inthis case the signal input circuit would be designed in the well knownmanner to be switched from adjustable tuning over the 550 to 1700kilocycle range to the 43 to 50 megacycle range. This would also be trueof any of the radio frequency amplifier stages preceding the signalinput circuit of the converter.

The tunable local oscillator tank circuit of the converter would, ofcourse, be constructed so as to be concurrently switched from adjustmentover that oscillation range which will produce the amplitude modulatedsignals of the intermediate frequency of 455 kilocycles to the higheroscillation band which will produce the frequency modulated signals oftheintermediate frequency of 4.3 'megacycles. In any event, it'is to beunderstood that, depending upon whether the stages preceding theconverter tube had been adjusted for amplitude modulated carrier wavereception or frequency modulated carrier wave reception,

the intermediate frequency energy produced in the converter output willeither be of the 455 kilocycle value or of the 4.3 megacycl'e value.

The converter output electrode is connected to a source of positivepotential through a path which includes the coil L1 and the coil Lcarranged in series. The lower end of coil La may be bypassed to groundby condenser C4 so as to provide a high frequency return path. Thereinvention is restricted to the c0up1ing network shown in Fig. 1.

Assuming that the receiver has been adjusted to receive amplitudemodulated carrier waves in the broadcast range of 550 to 1700kilocycles, it will be understood, that intermediate frequency energy of455 kilocycles is generated in the output circuit of the converter. Insuch case the circuits C1L1 and 02-42; are ineffective at the 455kilocycle frequency, and the coupling network between the converter andthe first intermediate frequency amplifier tube appears as a signalresonant circuit tuned to the lower inter mediate frequency. In otherwords, the coupling network appears at the lower intermediate frequencyas if there exists but the circuit C3-La. In Fig. 2 there is shown theresponse curve of thecircuit La-Ca when amplitude modulated carriersignals are received. It will be noted that the resonance curvehas awidth of 10 kilocycles, and the curve is sufficiently broad to givesubstantial transmission of the various components of the amplitudemodulated carrier wave.

When the receiver is adjusted to receive frequency modulated carrierwaves, however, the

circuit CaLa presents a capacitative impedance to the 4.3 megacycleintermediate frequency energy. In otherv words the coil La acts as ahigh impedance element while condenser Ca acts as a capacity couplingelement between circuits C1--Li and Ila-02. It will, therefore, be seenthat C3 acts in the manner of a capacity coupling for a .band passnetwork. The magnitude of is arranged in shunt with coil Ll a condenserC1, the low potential end of the condenser being established at groundpotential. In shunt with coil In there is arranged a condenser C3. Thesignal input grid of the first intermediate fre' quency amplifier tubeis arranged for connection to the junction of coils L1 and Lo through apath which includes the direct current blocking condenser C5 and thecoil L2 arranged in series.

The condenser C2 is arranged in shunt with coil La, the low potentialand of the condenser bein at ground potential. Lei-C2 is tuned to thecenter frequency of 4.3 megacycles in the same manner as circuit Li-Ci.

Of course, between the first intermediate fre-'v quency amplifier tubeand the second amplifier tube there may beutilized an additional network.of the type shown in Fig. 1. Again, the second of circuit and use thegrid circuit of the limiter stage as a diode demodulation network. Inthe 'latter case this is possible because the limiter When regridcircuit includes a resistor in the low potential side of the gridcircuit, which'is shunted by an intermediate frequency by passcondenser. It is not believed necessary to describe the details of ,the'receiver any further, since the present capacity C: is so chosen at thehigher intermediate frequency that it will provide" sufficient couplingbetween the tuned circuits C1-L1 and L2C2 to impart the substantiallyfiat-topped resonance curve shown in Fig. 3. In the latter 'it will benoted that theb'and pass curve has a width of 200 kilocycles, while thetopof the curve is substantially flat. This is a most'desirablecharacteristic for the wide band, frequency modulatedcarrier waves,since substantially all the frequency components may be transmitted tothe intermediate frequency ampliflerl tube with minimum attenuation.

Of course, it is important to choose the con-' stants for Ca and La suchthat the circuit Is-Ca will be properly tuned to the'455 kilocycles andhave a satisfactory response curve at that frequency, and yet'provide acoupling capacity at the 14.3 megacycle intermediate frequency toprovide the response characteristic shown in Fig. 3. The followingpurely illustrative constants are given for the coupling network;

C3=400 micro-microfarads (mmf.) Ci=30 mmf.

C2=30 mmf. Lc=305 microhenries (mh.) L1=44 mh. L2=44 mh.

The Q of" coil is is preferably of the'order of 40. This value will passall tones in a satisfactory manner when receiving amplitude modulatedcarrier waves.

. While I have indicated and described a system for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention'is by no means limited to the particular organizationshown and described, but that many modifications may be made withoutdepartingfrom the scope of my invention, as set forth in the appendedclaims. i

' high frequency carrier waves and a transmission tube, a couplingnetwork for transmitting said waves to said tube, said coupling networkcomprising a pair of parallel resonant circuits each tuned to a commoncarrier frequency, a third parallel resonant circuit electricallyconnected in common to said pair of resonant circuits, said thirdcircuit being tuned to a carrier frequency sufficiently different fromthe first named carrier frequency to cause said pair of circuits to beineffective at said third circuit frequency whereby the third circuitfunctions as the sole transmission circuit at its frequency, and saidthird resonant circuit including a capacitative reactive element whosemagnitude is such that it functions as a capacity coupling between saidpair of resonant circuits at said common carrier frequency to provide asubstantial band pass response curve.

2. In combination with a pair of parallel resonant circuits each tunedto the same carrier frequency, a third parallel resonant circuit commonto said pair of circuits, said third resonant circuit being tuned to asufficiently lower carrier prising a pair of parallel resonant circuitseach tuned to a common carrier frequency, a third parallel resonantcircuit electrically connected in common to said pair of resonantcircuits, said third circuit being tuned to a carrier frequencysufficiently lower than the first named carrier frequency to cause saidpair of circuits to be ineffective at said-lower frequency whereby thethird circuit acts as the sole transmission path between the source andtransmission tube at said lower frequency, and said third resonantcircuit including a capacitative reactive element whose magnitude issuch that it functions as a capacity coupling between said pair ofresonant circuits at said common carrier frequency to provide asubstantial band pass response curve, said source of-modulated carrierwaves being adapted to have its mean frequency alternatively of thevalue of said pair of resonant circuits or of the value of said thirdresonant circuit.

4. In combination with a pair of parallel resonant circuits each tunedto the same carrier frequency, a third resonant circuit common to saidpair of circuits, said third resonant circuit being tuned to asubstantially lower carrier frequency, and the third circuit including areactive element adapted to function as the sole reactive couplingbetween said pair of resonant circuits at the carrier frequency of thelatter,

said third resonant circuit consisting of a coil and capacity arrangedin shunt relation, and said capacity element functioning as saidreactive coupling, said capacity having a magnitude such that theresponse curve of the coupled pair of resonant circuits at said commonfrequency is a wide band pass curve, and said common frequency beingsufliciently higher than the third circuit frequency to cause said pairof circuits to be ineffective at the lower frequency, said thirdresonant circuit having a response curve which is substantially narrowerin width than the wide curve at said lower frequency. v

5. In combination with a source of modulated high frequency carrierwaves and a transmission tube, a coupling network for transmitting saidwaves to said tube, said coupling network comprising at least a pair ofresonant circuits each tuned to a common carrier frequency, a thirdresonant circuit electrically connected in common to said'pair ofresonant circuits, said third circuit being tuned to, a carrierfrequency substantially lower than the first named carrier frequency,said third resonant circuit including a condenser whose magnitude issuch that it functions as the sole coupling means between said pair ofresonant circuits at said common frestantially narrower response curvewidth at said lower frequency.

6. In combination with a source of modulated high frequency carrierwaves and a transmission waves to said tube, said coupling networkcomprising at least a pair of resonant circuits each tuned to a commoncarrier frequency, a third resonant circuit electrically connected incom-' mon' to said pair of resonant circuits, saidthird circuit beingtuned to a carrier frequency substantially lower than the first namedcarrier frequency, said third resonant circuit including a condenserwhose magnitude is such that it functions as the sole coupling meansbetween said pair of resonant circuits at said common frequency toprovide a substantial band pass response curve, said source of modulatedcarrier

